1. Field of the Invention
This invention relates generally to active carbon having a metal component and more particularly concerns a substantially uniform dispersion of a metal component in a porous carbon matrix having a high surface area and a method for making same.
2. Description of the Prior Art
It is known that the presence of metals in active carbon can greatly enhance the efficiency and selectivity of the active carbon when it is employed in catalytic, sorption, or filtering applications. Wennerberg et al., U.S. Pat. No. 4,082,694 disclose a high surface area active carbon material which has a cage-like structure exhibiting a microporosity which contributes to over 60 percent of its surface and which has an effective BET surface area of greater than about 2,300 square meters per gram and a bulk density greater than about 0.25 gram per cubic centimeter. Wennerberg et al., disclose a process for making such high surface area active carbon by first heating an agitated combination of solid potassium hydroxide containing between 2 and 25 weight percent water and a carbonaceous material comprising coal coke, petroleum coke or a mixture thereof below about 483.degree. C., then heating the resulting dehydrated product at a temperature between 705.degree. C. and 983.degree. C. to thereby form active carbon, and finally cooling the resulting activated product and removing essentially all of the inorganic material therefrom by water washing to form the high surface area active carbon end product. Wennerberg et al., U.S. Pat. Nos. 3,642,657 and 3,817,874 and Wennerberg, U.S. Pat. No. 3,726,808 disclose related methods for making high surface area active carbon products.
Attempts to incorporate metal compounds into activated carbon by conventional physical impregnation techniques have been problematical. One disadvantage with physical impregnation of activated carbon with metal compounds is that the small pores at the surface of the active carbon particles are inaccessible to liquid penetration and prevent penetration of the liquid, metal-containing impregnating solutions, thereby rendering impossible uniform and thorough impregnation of the carbon particles with metal. Furthermore, physical impregnation of the active carbon causes partial blocking of the pores of the carbon particles resulting in an appreciable reduction of the active surface area thereof. In addition, it is not possible to control to any large extent the total quantity of the metal applied to the active carbon particles by impregnation and its distribution on and in the carbon particles, with the end result that there is a substantial risk that the metal will crystallize and agglomerate in an undesirable manner on the carbon particles.
Several techniques have been proposed to overcome the problems associated with impregnating active carbon with metal compounds. For example, Dimitry, U.S. Pat. No. 3,886,093 discloses activated carbons having uniformly distributed active metal sites and a method for making such activated carbons. The method of Dimitry involves mixing an aqueous solution of a lignin salt with an aqueous solution of a transition metal salt to precipitate the transition metal and lignin as a metal lignate. The transition metal must be capable of forming a chemical bond with the lignin and in so doing precipitating the lignin from solution as a metal lignate. Dimitry discloses that the time required to complete the precipitation is less than one hour and that usually 30 minutes is sufficient for this purpose. According to Dimitry, suitably the wet metal lignate precipitate can then be dried in a spray drier. The precipitate is then carbonized at a temperature between 371.degree. C. and 983.degree. C. and finally activated at a temperature between 760.degree. C. and 1065.degree. C. Dimitry states that, although drying the metal lignate precipitate is not critial to form an activated carbon product, drying is necessary to form a high surface area end product. However, Dimitry gives neither a general disclosure nor a specific example of what it means by a "high surface" area for its end product. Dimitry states that the active metal sites are uniformly distributed throughout the activated carbon end product and presents an electron micrograph of an activated carbon end product magnified 5,700 times. However, from this relatively low magnification micrograph, the distribution of the active metal sites in the activated carbon end product is not readily apparent.
Furthermore, Siren, U.S. Pat. No. 4,242,226 states that the metal content in the active carbon which can be achieved by pyrolysis and activation of a metal lignate precipitate is much too low for the majority of fields of use and that it is difficult using such technique to predetermine the properties of the resulting metal-containing active carbon end product owing to the substantially undefined structure of the lignin. Siren disclosed an alternative technique in which a cation of calcium, magnesium, barium, aluminum, copper or a transition metal and an anionic group chemically bound to a polyhexose derivative are caused to react in solution, and the resulting product is precipitated either spontaneously or by adding a suitable precipitating agent. Siren discloses that, after separating the precipitate from solution, the precipitate can, if desired, be dried, for example, by spray drying. Thereafter the separated reaction product is pyrolyzed and activated using conventional techniques to form the activated carbon. In the method of Siren, suitably the polyhexose derivative employed comprises an acid polyhexose derivative and preferably the anionic groups of the polyhexose derivative comprise carboxylic acid groups, sulfonic acid groups or phosphoric acid groups. Preferably the polyhexose derivatives contain from 1 to 3 metal cations per hexose unit.
However, techniques such as those of Dimitry and Siren which require the occurrence of a chemical reaction between the metal cation and the carbonaceous anion in solution and the precipitation in solution of the resulting reaction product impose severe limitations on the metal-containing active carbon end products which can be obtained. For example, only those metals or metal compounds can be incorporated into the active carbon structures which are available in forms which can react chemically with the carbonaceous anion in solution and which thereby produce reaction products with either precipitate spontaneously or can be precipitated by the addition of a precipating agent to the solution. Furthermore, limitations are imposed on the amount of metal or metal compounds that can be incorporated into the active carbon matrix by the stoichiometry of the reaction between the metal cation with the carbonaceous anion in solution. In addition, limitations on the uniformity of the distribution of the metal or metal compounds in the active carbon end product are imposed by factors which are intrinsic to any process involving conventional precipitation of a salt from solution. Such factors include co-precipitation and post-precipitation as well as irregularities in the nature of the crystal formed and in the rate of crystal growth in solution as a result of the concentration of the salt being precipitated, the excess concentration of either the cationic or anionic portion of such salt, the solution temperature, the time period over which precipitation occurs, the presence, concentration and relative solubilities of other materials in solution, and the changes in any of these factors during the course of the precipitation process.